Explanation: While somewhat exaggerated (by the Imperial era, a Roman loss of 15,000 men was history-changing, while in the mid-Republic it was 'merely' devastating), the Romans put a great deal of effort into preserving and retrieving their standards - when one was lost, it was considered a major blow to Roman pride and prestige, and entire military campaigns were undertaken in hopes of retrieving lost standards. Follow the eagle!
Is that because by then they were using actual soldiers, instead of cheap peasant conscripts? What about later on, how bad would Hadrian or Antoninus Pius (yes, I know, no major battles) have been burnt by that loss?
In part, yes, the loss was more severe due to the Legions being better trained and equipped by that time - and a mostly volunteer force, which is harder to replenish. Hadrian and Antoninus Pius would have been similarly hard-hit. It's also harder to muster and deploy a force across an entire continent (and, especially, to manage those supply lines and borders simultanously) than in the days of the mid-Republic, when you could just look around the neighboring villages and drag off whatever young men you needed to muster another army to die to Hannibal.
As far as my memory goes, the only time Romans lost comparable forces during the period of the Principate was during their gruesome civil wars - namely, the Year of Four Emperors and Year of Five Emperors.
Not just the enemy ranks - into the teeth of a phalanx! Battle of Pydna
The Romans, when they attacked the Macedonian phalanx, were unable to force a passage, and Salvius, the commander of the Pelignians, snatched the standard of his company and hurled it in among the enemy. Then the Pelignians, since among the Italians it is an unnatural and flagrant thing to abandon a standard, rushed on towards the place where it was, and dreadful losses were inflicted and suffered on both sides.
Hell of a thing to impale yourself on 20 foot long pikes just to get a bit of ornamentation back!
There were other similar incidents like this, I think. Possibly even more with throwing, though I can't remember those off the top of my head. I do remember when Caesar's soldiers hesitated to disembark from their ships and face the British, a standard-bearer was the one who was the first to take the plunge, and the rest of the troops followed because losing their standard would have been an immense shame!